St. Francois Mountains

 ( Mountain Ranges Of Missouri ) 

The intrusive rocks of the area are composed of three types: subvolcanic rock , Ring dike and central .Sims, P. K., Eva B. Kisvarsanyi and G. B. Morey, 1987, Geology and Metallogeny of Archean and Proterozoic Basement Terranes in the Northern Midcontinent, U.S.A., USGS Bulletin 1819

The subvolcanic intrusives are similar in geochemistry to the associated rhyolite volcanics, which they intrude into. They are granite with Granophyre quartz, Perthite potassium feldspar, biotite and magnetite. They are intrusive into the rhyolites with development of fine grained granophyre at the contact. At depth they exhibit a coarse-grained rapakivi texture. The subvolcanic granites are the most widespread igneous rocks and were thought to have been covered with extensive volcanics that have been removed by erosion.

The ring intrusives are high silica bodies which were intruded along ring faults associated with caldera collapse. Rock types include trachyandesite, trachyte, syenite and amphibole - biotite granite. They are commonly porphyritic.

The central plutons are highly evolved two mica (contain both biotite and muscovite) granites. Distinctive accessory minerals include: fluorite, topaz, apatite, spinel, allanite, sphene and cassiterite. They are enriched in tin, lithium, beryllium, rubidium, barium, yttrium, niobium, uranium, thorium and fluorine and are referred to as "tin granites'. Their circular to oval shape in plan view is consistent with emplacement within resurgent calderas.

The exposed igneous rocks of the St. Francois are surrounded at depth by the slightly younger (~100 my) widely distributed igneous Spavinaw terrane. The Spavinaw rocks are intersected in drill core across southern Missouri, southern Illinois, northern Arkansas, southern Kansas and northeastern Oklahoma. The Spavinaw rocks occur in outcrop only near Spavinaw, Oklahoma. The rhyolites and Volcanic ash of the Spavinaw are essentially identical to the volcanics of the St. Francois mountains (the Washington County volcanics).

The Saint Francois Mountains were formed by igneous activity, whereas most of the surrounding Ozarks are developed on Paleozoic as a dissected plateau. The localized vertical relief was caused by erosion following Tectonic uplift during the Pennsylvanian and Permian periods produced by the Ouachita orogeny to the south. Elevations and Stratum dips in the Ozark structural dome generally radiate downward and outward away from the Saint Francois mountains.

These ancient reef complexes formed the localizing structures for the mineralizing fluids that resulted in the rich ore deposits of the area.

The igneous rocks of the Saint Francois Mountains are interpreted to be a series of caldera complexes, similar in scale to the Yellowstone Caldera complex. However, it is debated whether the igneous activity was related to a hotspot, like Yellowstone, or an ancient subduction zone.

Granite has been commercially quarried since 1869 in the vicinity of Elephant Rocks State Park, a tor with huge weathered granite boulders. The red architectural granite quarried in the area has been used in buildings in St. Louis and other areas in the country. It is currently marketed as Missouri Red Headstone.

The St. Francis River in this area is a whitewater stream in the spring when water levels are up, and it has hosted the Missouri Whitewater Championships annually since 1968.

• Geography of Missouri
• Missouri National and State Parks
• Taum Sauk pumped storage plant

• Unklesbay, A.G; & Vineyard, Jerry D. (1992). Missouri Geology — Three Billion Years of Volcanoes, Seas, Sediments, and Erosion. University of Missouri Press. .